1. Field of the Invention
The present invention relates to a Multimedia Broadcast/Multicast Service (MBMS). More particularly, the present invention relates to a method and an apparatus for informing User Equipments (UEs) in a cell dedicated channel (CELL_DCH) state of information on a specific preferred frequency when a session starts in a Frequency Layer Convergence (FLC) situation in which a MBMS is provided at the preferred frequency.
2. Description of the Related Art
At the present time, mobile communication systems using a wideband Code Division Multiple Access (CDMA) scheme is being developed for packet service communication for the transmission of mass storage data and multimedia broadcast/communication for the transmission of multimedia services other than conventional voice communication. Accordingly, in order to support the multimedia broadcast/communication, a MBMS service capable of providing a service to a plurality of UEs from one or a plurality of multimedia data sources has been discussed.
The term “MBMS service” typically refers to a service for transmitting the same multimedia data to a plurality of receivers through a wireless network. Herein, the receivers share one radio channel, so that radio transmission resources can be efficiently used. This type of MBMS service is a service capable of supporting multimedia transmission of realtime images and voices, still images, texts, and so on, while simultaneously providing voice and image data according to the multimedia transmission. This requires a large amount of resources. In a MBMS service, because the same data must be transmitted to a plurality of cells including users, a Point-to-Point (PP) connection or a Point-to-Multiple (PM) connection is used according to the number of users included in each cell.
FIG. 1 is an exemplary block diagram schematically illustrating nodes joining the providing of a MBMS service in a mobile communication network. FIG. 1 shows an exemplary case obtained by applying the MBMS service to a 3rd Generation Project Partnership (3GPP) system which corresponds to a standard of a 3rd generation asynchronous mobile communication network scheme based on a Global System for Mobile Communication network (GSM) and a General Packet Radio Services (GPRS).
Referring to FIG. 1, UEs 161 through 163, 171 and 172 represent user equipments or subscribers capable of receiving the MBMS service, cells 160 and 170 represent physical or logical service areas respectively controlled by base stations, in other words, node Bs, for transmitting MBMS-related data to the subscribers by wire. A Radio Network Controller (RNC) 140 controls the cells 160 and 170, selectively transmits multimedia data to a specific cell, and controls radio channels having been established in order to provide the MBMS service. The RNC 140 is connected to the UEs 161 through 163, 171 and 172 by a Radio Resource Control (RRC) interface.
The RNC 140 is connected to a Packet Switched or Packet Service (PS) network such as the Internet through a Serving GPRS Support Node (SGSN) 130. Communication between the RNC 140 and the PS network is performed by a PS signaling. Specifically, a connection between the RNC 140 and the SGSN 130 will be referred to as an Iu-PS interface. The SGSN 130 controls the MBMS-related service of each subscriber. Representatively, the SGSN 130 plays a role of managing service charge-related data of each subscriber, selectively transmitting multimedia data to the specific RNC 140, and so on.
A Transmit Network 120 may provide a communication channel between a Broadcast Multicast Service Center (BM-SC) 110 and the SGSN 130, and be connected to an external network through a Gateway GPRS Support Node (GGSN). The BM-SC 110 is a source of MBMS data and makes its responsible for the scheduling of the MBMS data.
The RNC 140 is connected to a Circuit Switched (CS) network through a Mobile Switching Center (MSC) 150. The CS network denotes a legacy voice-based communication network based on the connection. Communication between the RNC 140 and the MSC 150 is performed by a CS signaling. Specifically, a connection between the RNC 140 and the MSC 150 will be referred to as an Iu-CS interface.
MBMS data streams generated from the BM-SC 110 are transferred to the UEs 161 through 163, 171 and 172 via the Transmit Network 120, the SGSN 130, the RNC 140 and the cells 160 and 170.
Further, a plurality of SGSNs and a plurality of RNCs corresponding to each SGSN may exist for one MBMS service. Each SGSN selectively transmits data to the RNCs and each RNC selectively transmits data to each cell. To accomplish this, each node stores a list (in case of the SGSN, a list of RNCs, in case of the RNC, a list of cells) of lower nodes to which data streams must be transferred and selectively transmits MBMS data to only the nodes stored in the list.
A MBMS system supports FLC technology capable of allocating a Preferred Frequency Layer (PL) to each of available MBMS services and allowing UEs using the MBMS services to receive a desired MBMS service at the PL if possible. It is preferred that the PL is constant for a whole service area, but it may be established differently based on each area due to frequency policies or cell situations.
FIG. 2 is an exemplary diagram illustrating a conventional frequency layer structure under the FLC situation in a MBMS system.
In FIG. 2, three frequency layers 221 through 223 using different frequencies 1 through 3 spatially overlap, and a plurality of UEs 211 through 214 are stationary on the frequency layers 221 through 223 before the session of a MBMS service starts. The frequency layer 222 of the frequency layers 221 through 223 is set as a PL of the MBMS service.
A Core Network (CN) 201 represents a BM-SC, a transmit network, a MSC, a SGSN, and so on. A RNC 202 for managing the UEs 211 through 214 and a RNC 203 for managing the frequency layers 221 through 223 are connected to the CN 201. Further, a wire path 231 for connecting the CN 201 to the RNCs 202 and 203 will be referred to as an Iu interface, and a wire path 232 for connecting the RNC 202 to the RNC 203 will be referred to as an Iur interface. Herein, a node B is omitted, which exists between the RNC 203 and the UEs 211 through 214 in order to directly manage the frequency layers 221 through 223. The RNC 203 is a Drift RNC (DRNC) for directly managing the frequency layers 221 through 223 including the UEs 211 through 214 and the RNC 202 is a Serving RNC (SRNC) for managing communication of the UEs 211 through 214.
Because the UEs 211, 213 and 214 (Cell_DCH UEs) having received a service through a Dedicated Channel (DCH) do not read a MBMS Control Channel (MCCH), they cannot recognize PL information. Accordingly, it is necessary to provide technology by which the RNC 202 can inform the Cell_DCH UEs 211, 213 and 214 of the PL information through a Dedicated Control Channel (DCCH).